Both protocols are synchronous protocols. But in USART start and stop bits will be there. In SPI need to provide a delay when CS is asserted or de-asserted.
Which protocol provides max throughput?
I suggest you use the SPI module, from the timing perspective, the synchronous mode of USART module is very similar to that spi module, the USART synchronous mode includes ONE Start bit, one or two STOP bits, but the SPI does not require any additional bits such as Start/Stop bits.
I think the spi module can give max throughput.
The data sheet of LPC55S69 says that the maximum supported bit rate for USART master and slave synchronous mode is 10 Mbit/s. The SPI is 50MBit/S.
Hope it can help you
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